The present invention relates to an apparatus for dehydrating green veneer by using a pair of rotatable dehydrating rolls one of which is a toothed roll and passing a veneer sheet through a nip formed between the peripheries of the rolls for mechanically squeezing part of water contained in the veneer sheet. More specifically, the invention relates to an improvement in such type of veneer dehydrating apparatus.
A typical veneer dehydrating apparatus of the type which has a pair of rolls for mechanically squeezing part of water from veneer sheet is disclosed, for example, in the Laid-open Unexamined Japanese Patent Application Publication (Kokai) H7-186106. This apparatus is constructed to include a pair of rotatable dehydrating roll assemblies disposed one above the other with the axes thereof extending parallel to each other and spaced so that a nip is formed between the peripheral surfaces of the roll assemblies, through which a sheet of green wood veneer is passed. More specifically, the paired roll assemblies are spaced from each other such that the peripheral surfaces thereof define a clearance for the nip whose dimension as measured radially of the rolls is about 75 to 90 percent of the thickness of the veneer sheet to be dehydrated. One of the roll assemblies includes a plurality of axially aligned steel roll sections each having formed on its peripheral surface a number of tooth-like projections whose height as measured in radial direction of the roll assembly from the peripheral surface thereof is less than the above clearance and pierceable into veneer sheet to exert compressive force. A pair of adjacent roll sections makes a set of roll sections with a total axial length of about 280 mm and a roll back-up device is located in an annular groove between each two adjacent sets of roll sections. The other roll assembly includes a steel roll clad with covering made of elastic material such as urethane rubber and having a thickness of about 6 mm and a Shore A hardness of about HS60. The covering has a plurality of cuts or annular grooves at positions corresponding to the above grooves in the toothed roll assembly to receive therein similar back-up devices. The apparatus further includes a conveyer for feeding a veneer sheet toward the nip between the roll assemblies.
With such apparatus, clearance of the nip between the upper and lower roll assemblies may be reduced, for example, to about 60 percent of veneer sheet thickness so that the veneer sheet is compressed by a greater force in an attempt to improve the dehydrating efficiency. In handling a veneer sheet having therein a hard portion such as a knot, however, such arrangement of roll assemblies for increased compression has problems. That is, when a knotty veneer sheet is passed through the apparatus, the steel roll sections of the toothed roll assembly will remain rigid, while the elastic covering of the other roll assembly in contact with a knot in veneer sheet is compressed to be deformed radially inward and, simultaneously, other part of the elastic covering adjacent to the knot is also subjected to deformation by tens ion. Thus, the knotty portion in veneer sheet receives a reaction force of an excessive magnitude and is compressed accordingly, with the result that the knot my be broken. This may make the veneer sheet void at the knot or allow a crack to occur in the veneer sheet thereby causing the sheet itself to break along the crack. Apparently production of such defective veneer sheets will cause a decrease in veneer yield rate. If such a defective veneer sheets having a void or crack is used in the subsequent processes, it will seriously affect the quality of the resulting products such as plywood or LVL boards.
Additionally, repeated compression of the elastic material during dehydrating operation will generate heat within the covering. Since the thermal conductivity of urethane rubber is rather low, the heat cannot be radiated readily, but accumulated within the covering. Such heat may cause the elastic covering to expand to such an extent that it is loosened and finally removed from the steel core shaft.
Covering of urethane rubber with a reduced hardness may be used to solve the above problems. With such covering, however, the urethane rubber itself is deformed excessively so that veneer sheet cannot be compressed sufficiently and, therefore, successful dehydration cannot be accomplished and the intended improvement in dehydrating efficiency cannot be achieved.
It is an object of the present invention, therefore, to provide a veneer dehydrating apparatus which can solve the above-identified problems.
To achieve the object of the invention, there is provided a veneer dehydrating apparatus having a pair of rotatable dehydrating roll assemblies disposed one above the other and having their axes extending parallel to each other, at least one of which is positively driven. The paired roll assemblies includes a first roll assembly having formed on the peripheral surface thereof a number of tooth-like projections extending radially outward from the peripheral surface and a second roll assembly having a steel core shaft which is clad with covering of elastic material such as urethane rubber with a predetermined thickness. The axes of the roll assemblies are spaced radially so as to form a clearance or a nip between the peripheral surfaces thereof which is smaller than the thickness of the veneer sheet to be dehydrated. The apparatus further includes a conveyer for feeding sheets of veneer successively into the nip between the roll assemblies.
The second roll assembly has formed therein a number of annular grooves spaced axially of the second roll assembly at an interval of 50 mm or less and each having a width of 10 mm or less. Thus, the second roll assembly has a number of sectional elastic portions which are separated by any two adjacent annular grooves. Each of such elastic portion is deformable independently of other similar portions.
When a veneer sheet having therein a hard portion such as a knot is being passed through the nip between the roll assemblies, elastic portions then adjacent to the knot are deformed by the compressive force exerted by the knot. The deformation occurs in such a way that the elastic portions reduce slightly their radial dimension while expanding outward and that such expansion is taken up by annular grooves. Thus, the deformation of the elastic portions 33 can occur more easily and hence the reaction force of the sectioned elastic portions acting on the knot is less than heretofore. Therefore, the knot is less susceptible to breakage, with the result that the aforementioned drawbacks and problems can be prevented successfully.
Each of the dimensions associated with the annular grooves, such as width and depth of each groove, interval at which the grooves are spaced, hardness of the elastic material for the covering, and the total diameter of the anvil roll assembly including the thickness of elastic covering, may be determined as required for the best results through experiment. For information, the description of the invention contains some examples of conditions under which good results were achieved.
The above and other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description of a preferred embodiment of the veneer dehydrating apparatus according to the present invention, which description will be made with reference to the accompanying drawings, wherein: